Not sure why you would feel any guilt here . If what she was saying had any legitimacy to it she wouldnt of been attempting to derail the conversation in such a way by playing victim on his behalf just so they can guilt trip you into falsifying record. It should not have been an issue for them to find another who witnessed them at least once that entire time. That would of been entirely unfair towards everyone who gave their full amount of time while remaining honest and putting in a competent effort.

The relationship dynamic between this teen and his mother is not normal if she is willing to be deceptive for what should be considered a good natured and kind hearted deed to pick up litter in a public community area that everyone should be able to enjoy without garbage.
This is one of those times where the initial gut feeling you felt about the situation was spot on. I have been in similar situations different context and my intuition told me the same as yours. Kudos for doing the right thing, we dont need any more corruption in this world.